System for automated focus measuring of a lithography tool

ABSTRACT

A system and method are used to calibrate a focus portion of an exposure section of a lithography tool. A wafer is exposed so that a resulted or formed patterned image is tilted with respect to the wafer. The tilting can be imposed based on controlling a wafer stage to tilt the wafer or a reticle stage to tilt the reticle. The wafer is developed. Characteristics of the tilted patterned image are measured with a portion of the lithography tool to determine focus parameters of an exposure system. The portion can be an alignment system. The measuring step can measure band width and/or band location of the tilted patterned image. Sometimes, more than one patterned image is formed on the wafer, then the measuring step can measure distance between bands and shifting of the bands with respect to a central axis of the wafer. The focus parameters can be focus tilt errors and/or focus offset. The focus parameters are used to perform calibration. Calibration is done by either automatically or manually entering compensation values for the measured focus parameters into the lithography tool.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Ser. No. 10/301,627, filedNov. 22, 2002 (now U.S. Pat. No. 6,885,429 that issued Apr. 26, 2005),which claims priority under 35 U.S.C. § 119(e) to U.S. ProvisionalPatent Application Ser. No. 60/391,93 1, filed Jun. 28, 2002, which areincorporated by reference herein in their entireties.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to a system and method for automatedmeasuring of a focus portion of an exposure section of a lithographytool.

2. Background Art

Lithography systems are routinely calibrated to ensure the exposureoptics are focused properly on a wafer surface. This decreasesfabrication of devices that do not comply with certain tolerances byreducing printing or imaging errors caused by focus offset or focus tilterrors. One method of calibrating focus optics involves using a focustest reticle with structures that are patterned into a layer ofphotoresist on a wafer. The patterned structures are then evaluatedmanually using a microscope and recorded data is input into aspreadsheet to determine calibration compensation values. These valuesare used to calibrate the exposure optics. Typically, this method causesseveral hours of downtime a week while an operator fabricates the testwafer, analyzes if under the microscope, and enters the data. Thus, thismethod is time consuming, costly in terms of lost production time, andprone to errors due to reliance on human judgement. Other methodsrequire the use of costly external equipment to generate calibrationvalues.

Therefore, what is needed is a system and method to calibratelithography tools by automating measurement of focus offset and focustilt using an existing lithography system (e.g., a stepper alignmentsystem) to evaluate a patterned structure or patterned structures.

BRIEF SUMMARY OF THE INVENTION

Embodiments of the present invention provide a method of calibrating alithography tool. The method includes the step of exposing a wafer sothat a resultant patterned image is tilted with respect to the wafer.The method also includes the step of developing the wafer. The methodalso includes the step of measuring characteristics of the tiltedpatterned image with a portion of the lithography tool to determinefocus parameters of an exposure system. The method also includes thestep of using the focus parameters to perform the calibrating step.

Other embodiments of the present invention provide a method includingthe step of exposing a wafer positioned at an angle so as to create aband-shaped pattern on the wafer that is tilted with respect to thewafer. The method also includes the step of developing the wafer. Themethod also includes the step of measuring characteristics of thepatterned image with a wafer alignment system. The method also includesthe step of determining focus parameters based on the measuring step.The method also includes the step of calibrating a focus portion of anexposure section of a lithography tool based on the determining step.

Still other embodiments of the present invention provide a method thatincludes the step of stepping a wafer along a predetermined axis. Themethod also includes the step of exposing the wafer at two or morepoints along the predetermined axis so that formed patterned images aretilted with respect to the wafer. The method also includes the step ofdeveloping the wafer. The method also includes the step of measuringcharacteristics of the patterned images with a wafer alignment system.The method also includes the step of determining focus parameters basedon the measuring step. The method also includes the step of calibratinga focus portion of an exposure section of a lithography tool based onthe determining step.

Advantages of the above embodiments are they only require one or morerepeat exposures, and then the wafer alignment system of an exposuretool is used to measure the patterned structure or structures as opposedto a human or external equipment. No external measurement is requiredand opportunity for human error is eliminated. This saves both time andmoney.

Another advantage of at least some of the embodiments is that thepatterns are printed at a tilt with respect to a wafer, which allowsfocus conditions during exposure to be monitored. These systems inmethod save both time and money, and virtually eliminate the measuringerrors caused by human judgments.

Another advantage of at least some of the above embodiments is that byperforming multiple exposures, multiple bands are produced, which allowsfor a Theta X (Tx) measurement as well as a Z measurement in a scanninglithography tool. The method can also be extended to monitor steppercharacterization, of Z, Tx and Ty focus axis.

Further embodiments, features, and advantages of the present inventions,as well as the structure and operation of the various embodiments of thepresent invention, are described in detail below with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

The accompanying drawings, which are incorporated herein and form a partof the specification, illustrate the present invention and, togetherwith the description, further serve to explain the principles of theinvention and to enable a person skilled in the pertinent art to makeand use the invention.

FIG. 1 shows an initial position of a wafer with respect to a reticleimage plane according to embodiments of the present invention.

FIG. 2A shows a side view of a tilted wafer after being exposed with aband according to embodiments of the present invention.

FIG. 2B shows a front view of the wafer of FIG. 2A.

FIG. 3 shows a flow chart of a method for forming a band on a waferaccording to embodiments of the present invention.

FIG. 4A shows a side view of a tilted wafer after being exposed with aband according to embodiments of the present invention.

FIG. 4B shows a top view of the wafer in FIG. 4A.

FIG. 5A shows a side view of a tilted wafer after being exposed with aband according to embodiments of the present invention.

FIG. 5B shows a top view of the wafer in FIG. 5A.

FIG. 6A shows a side view of a wafer at two exposure positions afterbeing exposed at each position with a band according to embodiments ofthe present invention.

FIG. 6B shows a top view of the wafer of FIG. 6A.

FIG. 6C shows a scanning beam that scans the wafer in FIGS. 6A and 6B.

FIG. 6D shows a graph generated from scanning using a lithography tool'salignment system beam to determine characteristics of patterns on thewafer in FIGS. 6A, 6B, and 6C according to embodiments of the presentinvention.

FIG. 7A shows a side view of a wafer at two exposure positions afterbeing exposed at each position with a band according to embodiments ofthe present invention.

FIG. 7B shows a top view of the wafer of FIG. 7A.

FIG. 8A shows a side view of a wafer at two exposure positions afterbeing exposed at each position with a band according to embodiments ofthe present invention.

FIG. 8B shows a top view of the wafer of FIG. 8A.

FIG. 9 shows a flow chart depicting a calibration method according toembodiments of the present invention.

FIG. 10 shows a flow chart depicting a calibration method according toembodiments of the present invention.

FIG. 11 shows a flow chart depicting a calibration method according toembodiments of the present invention.

FIG. 12 shows a portion of a lithography system used to calibrateexposure or projection optics.

The present invention will now be described with reference to theaccompanying drawings. In the drawings, like reference numbers indicateidentical or functionally similar elements. Additionally, the left-mostdigit(s) of a reference number identifies the drawing in which thereference number first appears.

DETAILED DESCRIPTION OF THE INVENTION

Overview

According to the present invention, a reticle is populated withidentical structures possessing a critical dimension (CD) near aresolution capability of a stepper being used. The structures aredensely spaced, covering the entire exposure field. The reticle isexposed such that the image formed by the exposure tool's projectionoptics is tilted with respect to a wafer. The result of exposing thereticle image onto a tilted wafer is that only a portion of the reticleimage will resolve upon the wafer within the usable depth of focus(UDOF) of the exposure tool. Within the UDOF, the structures will printin the photoresist. However, the structures falling outside the UDOFwill not adequately resolve and the incident light will be adequate toclear away all of the photoresist. After proceeding through a developprocess, a visible band of patterned photoresist will remain on thewafer.

When resist properties and exposure conditions remain constantcharacteristics of the system can be determined based on a band locationand width. The location of the band is primarily a function of the focusoffset and tilt. The width of the band is dependent on tilt, UDOF,resist sensitivity, exposure dose, and other factors. The contrastbetween the patterned band and the bare silicon wafer can be detected bya wafer alignment system. After the development process, the wafer ispositioned back into the exposure tool and the precise locations of thepatterned bands are established by the wafer alignment system. Thelocations of the bands can be used to calculate the focus offset andfocus tilt errors that were present during the exposure step. Themethods can also be extended to monitor stepper characterization, of Z,Tx and Ty focus errors.

FIG. 1 shows a portion 100 of an exposure section of a lithography toolaccording to embodiments of the present invention. FIG. 1 shows a sideview of a field of view (e.g., a field or a surface) on a wafer 102where an image plane 104 formed by projection optics 106 is properlyfocused. In this configuration, an image 108 prints properly over anentire length of a slot (not shown) of a reticle (not shown) ontosubstantially an entire length of the field.

As described below, a tilted grating monitoring method according toembodiments of the present invention relies on a basic principle oflithography that small features will not print and resolve when printedout of focus. This principle applies whether the defocus is positive ornegative. Normally, wafer 102 is oriented to be in plane with imageplane 104. For the purposes of discussion, an example employing alithographic scanner will be described. However the same technique maybe extended to analyze focus performance on any other lithography tool.

It is to be appreciated, exposing of a multiple test band or band on awafer either in plane or tilted with respect to a focal plane of thefocus optics can be performed and used for other lithography reasons.For example, to view a field on a wafer and to determine variouscharacteristics and/or parameters of the field or the wafer for manydifferent lithographic related reasons.

Calibrating a Focus Portion Based on Measuring Focus Parameters with OnePatterned Band

FIGS. 2A and 2B show a wafer 200 according to embodiments of the presentinvention. Wafer 200 is purposely tilted so that areas 202 of the fieldof wafer 200 that are out of focus have no printed features. Thepresence of printed features only in a center of the field of the wafer200 results in a band 204. Thus, areas 202 have no features or resolvedor printed because they are out of focus. As shown by orienting axes inFIGS. 2A and 2B, an X-axis runs into the page in FIG. 2A, as does thescanned exposure that runs along the X-axis. Introducing a tilt aboutthe X-axis allows a center 206 of the field to remain in focus, whilethe top and bottom edges (e.g., areas 202) of the field go out of focus.Given a limited depth of focus, fine features do not resolve well at thetop and bottom (e.g., areas 202) of the printed field.

FIG. 3 shows a flow chart depicting a tilted grating monitoring method300 according to embodiments of the present invention. At step 302, awafer (e.g., wafer 200) is tilted Theta X (Θx) (shown in FIG. 2A) from anominal image plane (e.g., plane 104) in an exposure section. At step304, a grating reticle (e.g., a reticle filled with fine features) ispositioned to expose the wafer. At step 306, a field (e.g., a surface ofwafer 200) is exposed during scanning of the wafer along the X-axis. Thetilt causes the top and bottom regions (e.g., areas 202) of each fieldto be printed out of focus. This results in the small lines of thegrating not resolving at the top and bottom (e.g., areas 202) regions ofthe field when the resist (e.g., positive resist) in these regions ofthe field get cleared away. After clearing away these regions, a band(e.g., band 204) remains in a center potion (e.g., portion 206) of thefield. At step 308, a development process is performed on the wafer toclear resist from regions (e.g., areas 202) where the image did notprint due to defocus.

With reference again to FIGS. 2A and 2B, the characteristic of printedband 204 allows for some characterization of focus performance. Forexample, how precisely band 204 is centered along a Y-axis of the fieldis dependent on how well a focus system (not shown) established Z whileexposing. Also, how precisely width W of band 204 is generated isrelated to how well the focus systems control Theta-X. Thus, bymeasuring these characteristics of band 204 with an alignment systemsmetrology system, compensation values are determined and used tocalibrate the exposure optics.

FIGS. 4A and 4B show a wafer 400 according to embodiments of the presentinvention. In these figures, wafer 400 is created with the same Theta-Xtilt as FIGS. 2A and 2B, but a Z focus offset is introduced. This changemoves a printed band 402 away from a central X-axis. The Z focus offsethas shifted the in-focus region from a center 404 to an edge 406 of thefield. The relationship between band position and the Z offset is afunction of the induced Theta-X tilt.

FIGS. 5A and 5B show a wafer 500 according to embodiments of the presentinvention. Wafer 500 has been exposed with Theta X offset from nominal,which affects a width W of band 502. A vertical width W of printed band502 can also be affected by process parameters, such as dose, resistthickness, and resists choice among others. For example, width W1 may bea nominal width and width W2 may be a width caused by the above factors.This makes it difficult to establish a scale factor and nominal readingsfor Theta-X to band size. To overcome this difficulty, the embodimentsdiscussed below expose and measure multiple bands, with two bands usedmerely as an example embodiment.

Calibrating a Focus Portion Based on Measuring Focus Parameters withMultiple Patterned Bands

FIGS. 6A and 6B show two exposure positions for a wafer 600 according toembodiments of the present invention. Wafer 600 includes two patternareas or bands 602 and 604. Band 602 is formed when wafer 600 ispositioned at Z offset 2 and band 604 is formed when wafer 600 ispositioned at Z offset 1. FIG. 6C shows a scanning beam 606, which cancome from an alignment system in the lithography tool, that scans alonga Y axis to detect bands 602 and 604. FIG. 6D shows a graph generatedfrom detection of reflected light from wafer 600. As is shown, bands 602and 604 cause higher levels of reflected intensity 608 and 610,respectively. This can be preferably based on reflectivity effects oflight, although scattering effects can also be detected. Thereflectivity and/or scattering effects of light are based on the lighthitting a rough surface (e.g., bands 602 and 604) compared to justreflecting from a smooth or relatively smoother surface (e.g.,unpatterned areas of wafer 600).

FIGS. 7A and 7B show two exposure positions for a wafer 700 according toembodiments of the present invention. Wafer 700 includes two patternareas or bands 702 and 704. Band 702 is formed when wafer 700 ispositioned at Z offset 2 and band 704 is formed when wafer 700 ispositioned at Z offset 1. The wafer position for wafer 600 in FIGS. 6Aand 6B is shown in phantom in FIG. 7A. By moving wafer 700 a distance Zshift from a position of wafer 600, bands 702 and 704 are shiftedupwards Y1 change and Y2 change, respectively, on wafer 700.

FIGS. 8A and 8B show two exposure positions for a wafer 800 according toembodiments of the present invention. Wafer 800 includes two patternareas or bands 802 and 804. Band 802 is formed when wafer 800 ispositioned at Z offset 2 and band 804 is formed when wafer 800 ispositioned at Z offset 1. The wafer position for wafer 600 in FIGS. 6Aand 6B is shown in phantom. Thus, by rotating wafer 800 a distanceTx-shift from a position of wafer 600, bands 802 and 804 are shiftedupwards Y1-change and downward Y2-change, respectively, on wafer 800.

FIGS. 6-8 show the use of a double tilted grating technique that helpsto uncouple Z and Theta-X from the issues that affect band size, asdiscussed above with respect to single band wafers. Although theseembodiments are shown with two bands, it is to be appreciated any numberof parallel bands can be formed and measured without departing from thescope of the invention. In embodiments showing the double tilted gratingmethod, the field is exposed twice. The same induced Theta-X tilt isused for both exposures. However, a different Z offset is introduced oneach exposure in order to move the resulting grating to the top andbottom of the field. This results in two bands being printed. Thus, thedouble band tilted grating test uses a Z focus offset in combinationwith a known Theta X tilt to print a set of grating bands that is offsetfrom the center of the field. In the embodiments shown, each field isexposed twice, once with a positive Z offset and once with a negative Zoffset. The result is two bands, one in the top half of the field andone in the bottom half of the field.

In some embodiments, the following formulas are used to determine Z andTheta X. It is to be appreciated, other known formulas for determiningthese values can also be used. The location of the center of each bandin the field (such as shown in FIG. 6B) is referred to as Y Position,which can be defined by the following formula:

${YPosition} = \frac{ZOffset}{{Sin}({ThetaX})}$

For small angles, Sin(θ)≈θ so the equation simplifies to:

${YPosition} = \frac{ZOffset}{ThetaX}$

If system focus errors are incorporated, the equation becomes:

${YPosition} = \frac{{ZOffset} + {\Delta\; Z}}{{ThetaX} + {\Delta\;{Tx}}}$

Where ΔZ and ΔTx are the system focus errors. Creating an equation foreach of the two bands results in:

$Y_{1} = {{\frac{Z_{1} + {\Delta\; Z}}{{Tx} + {\Delta\;{Tx}}}\mspace{59mu} Y_{2}} = \frac{Z_{2} + {\Delta\; Z}}{{Tx} + {\Delta\;{Tx}}}}$

Notice that the same system focus errors and Tx offset apply to bothbands. Solving the first equation for ΔZ and the second for ΔTx gives:

Δ Z = Y₁(Tx + Δ Tx) − Z₁${\Delta\;{Tx}} = {\frac{Z_{2} + {\Delta\; Z}}{Y_{2}} - {Tx}}$

Substituting one into the other and solving gives us our final equationsfor the system focus errors:

${\Delta\; Z} = \frac{{Y_{1} \cdot Z_{2}} - {Y_{2} \cdot Z_{1}}}{Y_{2} - Y_{1}}$${\Delta\;{Tx}} = {\frac{Z_{2} - \; Z_{1}}{Y_{2} - Y_{1}} - {Tx}}$

In these formulas, the delta Z and Tx system focus errors are notaffected by the width of the bands. Therefore, they are independent ofprocess effects, to a first order approximation.

In some embodiments, the location of the bands is determined with anative alignment system in a lithographic exposure tool. As discussedabove, by scanning an illumination beam (FIG. 6C) along the Y axis andmeasuring the diffraction efficiency or reflectivity of the wafersurface, the locations of the bands can be measured (FIG. 6D). In thisfashion, one can expose a double tilted grating, develop the wafer, andhave it scanned by the lithographic exposure tool to assess one or morefocus performance factors in a relatively quick fashion. Then, thesefactors can be easily used to generate compensation values to calibratethe lithography tool, as discussed above.

Methods of Calibrating Focus

FIG. 9 shows a flow chart depicting a method 900 according toembodiments of the present invention. At step 902, a wafer is exposed sothat a patterned image is tilted with respect to the wafer. In apreferred embodiment, the tilting can be imposed based on controlling awafer stage to tilt the wafer. Other embodiments include controlling thereticle stage to tilt the reticle. At step 904, the wafer is developed.At step 906, characteristics of the tilted patterned image are measuredwith a portion of the lithography tool to determine focus parameters ofan exposure system. The measuring step can measure band width and/orband location of the tilted patterned image. The focus parameters can befocus tilt errors and/or focus offset, for example. At step 908, thefocus parameters are used to perform calibration. Calibration is done byeither automatically or manually entering compensation values for themeasured focus parameters into the lithography tool.

FIG. 10 shows a flow chart depicting a method 1000 according toembodiments of the present invention. At step 1002, a wafer is exposedso that a patterned image is tilted with respect to the wafer. Thetilting can be imposed based on controlling a reticle stage, forexample. At step 1004, the wafer is developed. At step 1006,characteristics of the patterned image are measured with a waferalignment system. The measuring step can measure band width and/or bandlocation of the tilted patterned image. At step 1008, focus parametersare determined based on the measuring step. The focus parameters can befocus tilt errors and/or focus offset, for example. At step 1010, afocus portion of an exposure section of a lithography tool is calibratedbased on the determining step. Calibration is done by eitherautomatically or manually entering compensation values for the measuredfocus parameters into the lithography tool.

FIG. 11 shows a flow chart depicting a method 1100 according toembodiments of the present invention. At step 1102, a wafer is steppedalong a predetermined axis. At step 1104, the wafer is exposed at two ormore points along the predetermined axis so that patterned images aretilted with respect to the wafer. The tilting can be imposed based oncontrolling a reticle stage, for example. At step 1106, the wafer isdeveloped. At step 1108, characteristics of the patterned images aremeasured with a wafer alignment system. The measuring step can measureband width and/or band location of the tilted patterned image. At step1110, focus parameters are measured based on said measuring step. Thefocus parameters can be focus tilt errors and/or focus offset, forexample. At step 1112, a focus portion of an exposure section of alithography tool is calibrated based on said determining step.Calibration is done by either automatically or manually enteringcompensation values for the measured focus parameters into thelithography tool.

Exemplary System

FIG. 12 shows a portion 1250 of a lithography system used to calibrateoptics 1252 (exposure or projection optics). Portion 1250 can be used toperform one or more of the calibration methods discussed above inregards to FIGS. 1–11. Portion 1250 comprises a reticle 1254 on areticle stage 1256, optics 1252, a wafer 1200 on a wafer stage 1258, analignment system 1260, and a calibration value determining system 1262.In one example, alignment system 1260 comprises a metrology system 1264.As discussed above, alignment system 1260 or metrology system 1264 isused to measure features formed on wafer 1200, as discussed above, toproduce a measured signal 1266. Measured signal 1266 is processed incalibration value determining system 1262 to form a calibration signal1268. Calibration signal 1268 is used to calibrate optics 1252. Thus,portion 1250 allows for automated calibration of optics 1252.

Conclusion

While various embodiments of the present invention have been describedabove, it should be understood that they have been presented by way ofexample only, and not limitation. It will be apparent to persons skilledin the relevant art that various changes in form and detail can be madetherein without departing from the spirit and scope of the invention.Thus, the breadth and scope of the present invention should not belimited by any of the above-described exemplary embodiments, but shouldbe defined only in accordance with the following claims and theirequivalents.

1. A lithographic apparatus, comprising: means for exposing a wafer sothat a resultant patterned image is tilted with respect to the wafer; ametrology system that performs an automated measurement ofcharacteristics of the tilted patterned image to determine focusparameters of an exposure system; and a calibration device thatautomatically calibrates the lithographic apparatus using the focusparameters from the automated measurement.
 2. The lithographic apparatusof claim 1, wherein the metrology system measures focus tilt errors ofthe exposure system.
 3. The lithographic apparatus of claim 1, whereinthe metrology system measures focus offset of the exposure system. 4.The lithographic apparatus of claim 1, wherein the metrology systemmeasures a band location of the tilted patterned image.
 5. Thelithographic apparatus of claim 1, further comprising: a reticle stagethat tilts a reticle to impose the tilt in the resultant patternedimage.
 6. The lithographic apparatus of claim 1, further comprising: awafer stage that moves the wafer along a predetermined axis during anexposing operation to form at least one additional tilted patternedimage on the wafer.
 7. The lithographic apparatus of claim 6, whereinthe metrology system measures a distance between the tilted patternedimages.
 8. The lithographic apparatus of claim 6, wherein the metrologysystem measures a distance of a center axis of the tilted patternedimages from a center axis of the wafer.
 9. A lithographic apparatus,comprising: a means for exposing a wafer positioned at an angle so as tocreate a band-shaped etch pattern on the wafer that is tilted withrespect to the wafer; a wafer alignment system that measurescharacteristics of the patterned image and determines focus parameters;and a calibrating device that calibrates a focus portion of an exposuresection of a lithography apparatus based on the focus parameters. 10.The lithographic apparatus of claim 9, wherein wafer alignment systemdetermines focus tilt errors of the focus portion.
 11. The lithographicapparatus of claim 9, wherein wafer alignment system determines focusoffset of the focus portion.
 12. The lithographic apparatus of claim 9,wherein wafer alignment system measures a band location of the tiltedpatterned image.
 13. The lithographic apparatus of claim 9, furthercomprising: a reticle stage that tilts a reticle impose the tilt in theband-shaped etch pattern on the wafer.
 14. The lithographic apparatus ofclaim 9, further comprising: a wafer stage that moves the wafer along apredetermined axis during an exposing operation to form at least oneadditional tilted patterned image on the wafer.
 15. The lithographicapparatus of claim 14, wherein the wafer alignment system measures adistance between the tilted patterned images.
 16. The lithographicapparatus of claim 14, wherein the wafer alignment system measures adistance of a center axis of the tilted patterned images from a centeraxis of the wafer.